fix(CDVPlugin): Swift init

[dpogue]

Platforms affected

iOS

Motivation and Context

Fixes #1288

Description

This ensures that the default init method is called for CDVPlugin subclasses, to allow Swift initializers to run as expected.

Aside: I still feel like NS_DESIGNATED_INITIALIZER would be the theoretically correct way to solve this, but that doesn't work when our designated initializer with the webview is part of private implementation details (your designated initializer must be part of your public API surface). The risk is that someone could (in their own code) do [[MyCordovaPlugin alloc] init] and try to construct a plugin instance without providing the webview engine, and lead to all sorts of weird issues.

However, that problem already existed and people don't seem to be shooting themselves in the foot with it (or at least, not complaining here when they do), so it's probably better to fix the issue that is impacting people even if the solution isn't theoretically perfect.

Testing

Added a unit test which ensures that an init-time property in a Swift plugin is correctly set after the plugin has been initialized. This test case fails without the changes to CDVPlugin.m.

Checklist

• [ ] I've run the tests to see all new and existing tests pass
• [ ] I added automated test coverage as appropriate for this change
• [ ] Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• [ ] If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• [ ] I've updated the documentation if necessary

[codecov-commenter]

Codecov Report

Merging #1295 (39b3fd4) into master (2c86d1a) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1295   +/-   ##
=======================================
  Coverage   78.45%   78.45%           
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  Files          15       15           
  Lines        1778     1778           
=======================================
  Hits         1395     1395           
  Misses        383      383           

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